In many optical signal transmission applications, it is important for the optical source to exhibit excellent linearity and noise performance. One such application is in high-quality analog optical fiber systems for providing Community Antenna Television (CATV) service. In such a system, an analog CATV signal including several channels of video is modulated onto one or more optical carrier signals for distribution via optical fiber from a cable system head end. Conventional systems for transmitting analog CATV signals over optical fiber generally require highly-linear, low-noise lasers and/or optical modulators. These types of optical signal sources are usually very expensive, and their excessive cost has been a significant impediment to the widespread deployment of fiber-to-the-home transmission systems. The expensive optical signal sources include, for example, analog-grade distributed feedback (DFB) lasers, as well as continuous wave (CW) lasers, such as yttrium aluminum garnet (YAG) and yttrium lanthanum fluoride (YLF) lasers, which require external modulators.
Conventional optical transmission systems are described in, for example, U.S. Pat. No. 5,548,436 entitled "Optical Transmission System" and issued Aug. 20, 1996 to Ramachandran et al., U.S. Pat. No. 5,457,557 entitled "Low Cost Optical Fiber RF Signal Distribution System" and issued Oct. 10, 1995 to Zarem et al., U.S. Pat. No. 5,450,508 entitled "Apparatus and Method for Optical Fiber Alignment Using Adaptive Feedback Control Loop" and issued Sep. 12, 1995 to Decusatis et al., and U.S. Pat. No. 5,384,651 entitled "Optical Transmission System" and issued Jan. 24, 1995 to Van de Voorde et al. Unfortunately, none of these conventional systems are configured to permit a low-cost optical source, such as a digital-grade DFB laser diode or a Fabry-Perot laser diode, to be used in place of an expensive highly-linear, low-noise optical source.
A number of techniques for improving the linearity or noise performance of an optical source are known. These techniques include the use of pre-distortion circuits, and feedforward approaches in which a second optical source is used. However, conventional pre-distortion circuits generally correct for only source nonlinearity, and an alternative mechanism is therefore required to correct for the source noise. Also, pre-distortion circuits generally require numerous critical adjustments which increase the cost and complexity associated with the source. The second optical source used in conventional feedforward approaches not only involves considerable additional cost and complexity, but introduces wavelength control issues.
It is therefore apparent that a need exists for techniques which provide the linearity and noise performance required in analog signal transmission over optical fiber and other important applications, without the excessive cost and complexity associated with conventional highly-linear, low-noise optical sources.